Thursday, December 10, 2015

Ageing Hypergiant Star Expelling Large Dust Grains

VY Canis Majoris is an ageing red hypergiant star located approximately 3840 light years from Earth. In terms of size, VY Canis Majoris is one of the largest stars known, with approximately 1420 times the radius of the Sun. If placed in the Solar System, the surface of VY Canis Majoris would extend well beyond the orbit of Jupiter. VY Canis Majoris is estimated to contain up to 35 times the Sun’s mass and about 300,000 times the Sun’s luminosity.

Observations of VY Canis Majoris using the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument at the Very Large Telescope (VLT) at the European Southern Observatory’s Paranal Observatory in Chile show the presence of large dust grains in the thick stellar wind of VY Canis Majoris. Giant stars like VY Canis Majoris expel huge amounts of gas and dust. VY Canis Majoris expels ~30 times the mass of Earth in the form of gas and dust every year.

Measurements of how light from VY Canis Majoris is polarised by the surrounding dust grains show that the dust grains around VY Canis Majoris are relatively large, about 0.5 micrometers in size. Although that may appear small, it is approximately 50 times larger than the average size of dust grains in interstellar space. These dust grains are sufficiently large to be accelerated from VY Canis Majoris by the star’s own radiation, thereby contributing to the star’s prodigious rate of mass loss.

VY Canis Majoris will eventually explode as a supernova. The large size of the dust grains allow them to be accelerated sufficiently far from VY Canis Majoris, making them resistant to sublimation in the ensuring supernova explosion. The large dust grains, together with the heavy elements created during the supernova explosion will go on to form subsequent generations of stars and planets.

Scicluna et al. (2015), “Large dust grains in the wind of VY Canis Majoris”, arXiv:1511.07624 [astro-ph.SR]